1. Field of the Invention
The present invention relates to a hydrogen absorbing alloy electrode used as a negative electrode of an alkali secondary battery such as a nickel-hydrogen secondary battery, a method of fabricating the hydrogen absorbing alloy electrode, and an alkali secondary battery using the hydrogen absorbing alloy electrode, and is characterized in that a hydrogen absorbing alloy used for the hydrogen absorbing alloy electrode is modified, to improve the activity in the early stages of the hydrogen absorbing alloy electrode and the characteristics thereof at low temperature.
2. Description of the Related Art
A nickel-hydrogen secondary battery has been conventionally known as one example of an alkali secondary battery. In the nickel-hydrogen secondary battery, a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy has been generally used as its negative electrode.
Examples of the hydrogen absorbing alloy used for the negative electrode include a hydrogen absorbing alloy having a CaCu5-type crystal structure using Misch metal (Mm) which is a mixture of rare earth elements or a Laves type hydrogen absorbing alloy.
In each of the hydrogen absorbing alloys, however, a coating of an oxide or the like is generally formed on its surface by natural oxidation, for example. When a hydrogen absorbing alloy electrode is fabricated using such a hydrogen absorbing alloy, and the hydrogen absorbing alloy electrode is used as a negative electrode of the nickel-hydrogen secondary battery, the activity in the early stages of the hydrogen absorbing alloy is low, and hydrogen gas is not sufficiently absorbed in the hydrogen absorbing alloy. As a result, some problems arise. For example, the capacity in the early stages of the nickel-hydrogen secondary battery is decreased, and the internal pressure of the battery is increased by the hydrogen gas.
Therefore, in recent years, a method of immersing a hydrogen absorbing alloy in an acid solution such as hydrochloric acid, to remove a coating of an oxide on the surface of the hydrogen absorbing alloy has been proposed, as disclosed in Japanese Patent Laid-Open No. 225975/1993.
When the hydrogen absorbing alloy is thus immersed in the acid solution, to remove the coating of the oxide on the surface of the hydrogen absorbing alloy, some active portions appear on the surface of the hydrogen absorbing alloy.
However, the active portions thus appearing on the surface are oxidized again, whereby the activity in the early stages of the hydrogen absorbing alloy is not sufficiently improved, and the hydrogen gas is not sufficiently absorbed in the hydrogen absorbing alloy in the early stages. As a result, some problems still exist. For example, the capacity in the early stages of the battery is low, and the internal pressure of the battery is increased.
Furthermore, in the hydrogen absorbing alloy electrode using the conventional hydrogen absorbing alloy, the electrochemical catalytic capability thereof is not sufficient, resulting in inferior discharge characteristics in a case where it is used under low temperature.
An object of the present invention is to improve, in a hydrogen absorbing alloy electrode used as a negative electrode of an alkali secondary battery such as a nickel-hydrogen secondary battery, the activity in the early stages of the hydrogen absorbing alloy electrode which is used as the negative electrode.
Another object of the present invention is to simply obtain a hydrogen absorbing alloy electrode whose activity in the early stages is improved, resulting in increased charging and discharging characteristics.
Still another object of the present invention is to improve, in an alkali secondary battery using a hydrogen absorbing alloy electrode as its negative electrode, the discharge capacity thereof in the early stages and prevent the internal pressure of the battery from being increased.
A further object of the present invention is to obtain, in an alkali secondary battery using a hydrogen absorbing alloy electrode as its negative electrode, sufficient discharge characteristics even in a case where the battery is used under low temperature.
In a first hydrogen absorbing alloy electrode according to the present invention, a hydrogen absorbing alloy containing at least nickel, cobalt and aluminum is used. Letting a be the sum of the respective abundance ratios of cobalt atoms and aluminum atoms in a portion to a depth of 30 xc3x85 from the surface of the hydrogen absorbing alloy, and b be the sum of the respective abundance ratios of cobalt atoms and aluminum atoms in a bulk region inside the hydrogen absorbing alloy, conditions of a/bxe2x89xa71.30 are satisfied.
As in the first hydrogen absorbing alloy electrode, when more cobalt atoms and aluminum atoms exist on the surface of the hydrogen absorbing alloy, as compared with those in the bulk region inside the hydrogen absorbing alloy, the catalytic actions of the cobalt atoms and the aluminum atoms cause the activity of the hydrogen absorbing alloy electrode using the hydrogen absorbing alloy to be improved from the early stages and cause the electron conductivity thereof under low temperature to be improved.
When the first hydrogen absorbing alloy electrode is used as a negative electrode of an alkali secondary battery such as a nickel-hydrogen secondary battery, the emission of hydrogen gas in the early stages is restrained, so that the capacity in the early stages of the battery is increased, and the internal pressure of the battery is prevented from being increased. Further, the electrochemical catalytic capability of the hydrogen absorbing alloy electrode is improved.
In a second hydrogen absorbing alloy electrode according to the present invention, a hydrogen absorbing alloy containing at least nickel, cobalt, aluminum and manganese is used. Letting A be the sum of the respective abundance ratios of cobalt atoms, aluminum atoms and manganese atoms in a portion to a depth of 30 xc3x85 from the surface of the hydrogen absorbing alloy, and B be the sum of the respective abundance ratios of cobalt atoms, aluminum atoms and manganese atoms in a bulk region inside the hydrogen absorbing alloy, conditions of A/Bxe2x89xa71.20 are satisfied.
As in the second hydrogen absorbing alloy electrode, when more cobalt atoms, aluminum atoms and manganese atoms exist on the surface of the hydrogen absorbing alloy, as compared with those in the bulk region inside the hydrogen absorbing alloy, the catalytic actions of the cobalt atoms, the aluminum atoms and the manganese atoms cause the activity of the hydrogen absorbing alloy electrode using the hydrogen absorbing alloy to be improved from the early stages and cause the electrochemical catalytic capability thereof to be improved.
When the second hydrogen absorbing alloy electrode is used as a negative electrode of an alkali secondary battery such as a nickel-hydrogen secondary battery, the emission of hydrogen gas in the early stages is restrained, so that the capacity in the early stages of the battery is increased, and the internal pressure of the battery is prevented from being increased. Further, the discharge characteristics in a case where the battery is used under low temperature are also improved.
In a first method of fabricating a hydrogen absorbing alloy electrode according to the present invention, in fabricating a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy containing at least nickel, cobalt and aluminum, the hydrogen absorbing alloy is surface-treated in an acid solution to which 1 to 5% by weight of a cobalt compound and an aluminum compound per the weight of the hydrogen absorbing alloy are respectively added.
When the hydrogen absorbing alloy containing nickel, cobalt and aluminum is thus surface-treated in the acid solution to which the cobalt compound and the aluminum compound are added, active portions appear on the surface of the hydrogen absorbing alloy, and the active portions are protected by a protective film composed of CoAl2O4. Therefore, the active portions are prevented from being oxidized again, and the respective numbers of the cobalt atoms and the aluminum atoms on the surface of the hydrogen absorbing alloy are larger than those in the bulk region inside the hydrogen absorbing alloy.
When the amounts of cobalt compound and the aluminum compound which are added to the acid solution are respectively set in the range of 1 to 5% by weight per the weight of the hydrogen absorbing alloy, the above-mentioned first hydrogen absorbing alloy electrode in which the relationship between the sum a of the respective abundance ratios of cobalt atoms and aluminum atoms in the portion to a depth of 30 xc3x85 from the surface of the hydrogen absorbing alloy and the sum b of the respective abundance ratios of cobalt atoms and aluminum atoms in the bulk region inside the hydrogen absorbing alloy satisfies conditions of a/bxe2x89xa71.30 is obtained.
When the respective amounts of the cobalt compound and the aluminum compound which are added to the acid solution are less than the above-mentioned range, the respective numbers of the cobalt atoms and the aluminum atoms in the portion to a depth of 30 xc3x85 from the surface of the hydrogen absorbing alloy are decreased. On the other hand, if the respective amounts of the cobalt compound and the aluminum compound are too large, the cobalt atoms and the aluminum atoms do not remain on the surface of the hydrogen absorbing alloy. In either one of the cases, the hydrogen absorbing alloy electrode satisfying the conditions of a/bxe2x89xa71.30 is not obtained.
As the cobalt compound and the aluminum compound which are added to the acid solution, any compounds which can be dissolved in the acid solution may be used. Examples of the cobalt compound include cobalt chloride and cobalt hydroxide (including cobalt oxyhydroxide). Examples of the aluminum compound include aluminum chloride and aluminum hydroxide.
In a second method of fabricating a hydrogen absorbing alloy electrode according to the present invention, in fabricating a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy containing at least nickel, cobalt, aluminum and manganese, the hydrogen absorbing alloy is surface-treated in an acid solution to which 1 to 5% by weight of an aluminum compound per the weight of the hydrogen absorbing alloy is added.
When the hydrogen absorbing alloy containing nickel, cobalt, aluminum and manganese is thus surface-treated in the acid solution to which the aluminum compound is added, the respective numbers of the cobalt atoms and the aluminum atoms on the surface of the hydrogen absorbing alloy are larger than those in the bulk region inside the hydrogen absorbing alloy.
When the amount of the aluminum compound added to the acid solution is set in the range of 1 to 5% by weight per the weight of the hydrogen absorbing alloy, the above-mentioned second hydrogen absorbing alloy electrode in which the relationship between the sum A of the respective abundance ratios of cobalt atoms, aluminum atoms and manganese atoms in the portion to a depth of 30 xc3x85 from the surface of the hydrogen absorbing alloy and the sum B of the respective abundance ratios of cobalt atoms, aluminum atoms and the manganese atoms in the bulk region inside the hydrogen absorbing alloy satisfies conditions of A/Bxe2x89xa71.20 is obtained.
When the amount of the aluminum compound added to the acid solution is less than the above-mentioned range, the respective numbers of the cobalt atoms, the aluminum atoms and the manganese atoms in the portion to a depth of 30 xc3x85 from the surface of the hydrogen absorbing alloy are decreased. On the other hand, if the amount of the aluminum compound is too large, the cobalt atoms, the aluminum atoms and the manganese atoms do not remain on the surface of the hydrogen absorbing alloy. In either one of the cases, the hydrogen absorbing alloy electrode satisfying the conditions of A/Bxe2x89xa71.20 and a/bxe2x89xa71.3 is not obtained.
In each of the first and second methods of fabricating the hydrogen absorbing alloy electrode, if the pH of the acid solution is too high, a coating of an oxide or the like on the surface of the hydrogen absorbing alloy cannot be sufficiently removed. On the other hand, if the pH of the acid solution is too low, an active metal in the hydrogen absorbing alloy is dissolved, so that the number of active portions on the surface of the hydrogen absorbing alloy is decreased. Therefore, the initial pH of the acid solution is set preferably in the range of 0.7 to 2.0.
When the temperature of the acid solution is too high, the active metal in the hydrogen absorbing alloy is also dissolved, so that the number of the active portions on the surface of the hydrogen absorbing alloy is decreased. On the other hand, if the temperature of the acid solution is too low, the coating of the oxide or the like on the surface of the hydrogen absorbing alloy cannot be sufficiently removed. Therefore, the temperature of the acid solution is set preferably in the range of 20xc2x0 C. to 70xc2x0 C.
Furthermore, in treating the hydrogen absorbing alloy in the acid solution as described above, it is preferable that a quinone compound such as anthrahydroquinone is added to the acid solution. When the quinone compound is thus added to the acid solution, dissolved oxygen in the acid solution is removed, so that the active portions appearing on the surface of the hydrogen absorbing alloy are prevented from being oxidized again, and the activity in the early stages of the hydrogen absorbing alloy is further improved. The amount of the quinone compound added to the acid solution is preferably 5 ppm to 100 ppm.
The hydrogen absorbing alloy having a CaCu5-type crystal structure used in the present invention is represented by a general formula MmNiaCobAlcMnd. In the formula, Mm is a mixture of rare earth elements selected from La, Ce, Pr, Nd, Sm, Eu, Sc, Y, Pm, Gd, Tb, Gy, Ho, Er, Tm, Yb and Lu. Particularly, Mm mainly composed of a mixture of La, Ce, Pr, Nd and Sm is preferable. Further, a greater than 0, b greater than 0, c greater than 0, and dxe2x89xa70, and 4.4xe2x89xa6a+b+c+dxe2x89xa65.4.
The hydrogen absorbing alloy composed of the above-mentioned composition can satisfy the basic performance such as cycle characteristics and discharge characteristics of the alkali secondary battery. Further, elements Si, C, W and B may be added in the range in which the properties of absorbing hydrogen in the hydrogen absorbing alloy are not changed.
In the above-mentioned composition formula, it is preferable that the amount a of nickel is 2.8xe2x89xa6axe2x89xa65.2, the amount b of cobalt is 0 less than bxe2x89xa61.4, the amount c of aluminum is 0 less than cxe2x89xa61.2, and the amount d of manganese is dxe2x89xa61.2. Further, in order to increase the capacity of the battery, it is preferable that the amount c of aluminum is cxe2x89xa61.0, and the amount d of manganese is dxe2x89xa61.0.
There and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.